ESA has successfully launched its Cheops telescope on a mission to make detailed follow-up studies of distant exoplanets discovered by earlier space-faring telescopes, such as Kepler and TESS. The Soyuz-Fregat rocket that hefted Cheops into orbit also carried an Italian Earth Observation satellite and a trio of Cube Sats.

The CHaracterising ExOPlanet Satellite, (Cheops) is ESA’s first satellite dedicated to probing alien worlds orbiting distant stars. It will fix its gaze on stars that are already known to host planets that range in size between our Blue Marble and Neptune, and watch for the tiny dip in light that occurs when these worlds pass between their parent stars and the waiting telescope.

As a planet makes its transit, Cheops will take detailed measurements of the celestial body’s size using its high precision photometer.

By combining information on an exoplanet’s size with measurements of its mass collected by other observatories, planetary scientists can calculate the world’s mean density. With this known, they can model its composition and structure, and even shed light on whether it may be a suitable environment for the emergence of extraterrestrial life.

The solar-powered satellite will also watch for changes in how starlight is reflected by an exoplanet during multiple phases of its orbit, in a similar way to how a person on Earth perceives the phases of the Moon – crescent, full etc. These observations will hopefully offer insights on the processes that govern heat transfer from the day side to the night side, and so reveal certain characteristics of a planet’s atmosphere, such as the presence of clouds and even their composition.

Cheops can only view one transit at a time, and so the satellite’s handlers must be careful to coax the maximum science possible out of the telescope. Thankfully, since it will only be observing already-discovered worlds, scientists already know when and where to point Cheops to catch a transit. This allows the satellite to maximize the time spent characterizing exoplanets with minimal wasted time between observations.

Artist's impression of Cheops observing transiting exoplanets

ESA/ATG medialab

While the discoveries made by Cheops will be exciting in their own right, they will also be valuable for those selecting targets for the next generation of flagship observatories, such as the James Webb Space Telescope.

Cheops shared the rocket fairing of its Soyuz-Fregat host with an Italian Space Agency Cosmo-SkyMed Second Generation Earth-observation satellite, and three Cube Sats.

At 9:54 CET on Dec 18, Cheops and its comrades were thrust into the skies above ESA’s spaceport in Kourou, French Guiana. The first three stages of the launch vehicle performed perfectly, sequentially separating until only the Fregat upper-stage and satellite payload remained.

Fregat, which is a relatively new addition to the Soyuz launcher family, qualified in the year 2000 with the goal of extending the capabilities of the ageing Russian rocket.

The Italian-made Cosmo-SkyMed was the first satellite to be deployed 22 minutes and 44 seconds after launch. A little over two hours later, Cheops finally bid farewell to Fregat. The satellite's signal was picked up soon after by ESA's New Norcia station in Australia. At the time of writing the Cube Sats have yet to deploy.